1. Field of the Invention
The invention relates to a bidirectional dispersion compensator for two optical signals that are transmitted over an optical fiber in opposite directions.
2. Description of the Related Art
In the transmission of signals with broad wavelength bands, it has been noted that the transmission speed depends on the wavelength. The compensation of this effect, which is termed dispersion, is an absolute technical condition for achieving acceptable transmission characteristics for systems with data rates of 10 Gbit/s and greater. Currently, there are several possibilities for compensation: compensation fibers, fiber-BRAGG grids, or optical phase conjugation.
Special glass fibers are used as compensation fibers, the total dispersion of which is exactly as great as that of the transmission paths being compensated, but which have an inverted operational sign. These types of compensators are expensive and require voluminous system components; in addition, a significant polarization mode dispersion arises. The effect of this is that orthogonal polarization states comprise different transit times through the compensation fibers.
The U.S. Pat. No. 5,596,448 teaches a dispersion compensator 10 as represented in FIG. 1, which this utilizes a compensation fiber that is provided with a reflector, which fiber is traversed twice by optical signals that are to be compensated. The length of this compensation fiber is halved, and thus the costs and volume are appreciably reduced. Besides the compensation line (compensation fiber) LK and the reflector R, the dispersion compensator contains a circulator Z, which delivers the optical signal that enters at terminal Z.1 (TOR 1) at the next terminal, terminal Z.2, and in turn delivers at terminal Z.3 the reflected compensated signal that is fed in at terminal Z.2.
When the reflector is constructed as a Faraday rotator reflector (polarization conversion mirror) which converts the polarization state of the incoming light into the orthogonal polarization state for the reflected light, the polarization mode dispersion of the compensation line is eliminated.
The use of this dispersion compensator is appropriate when signals are transmitted over an optical fiber in one direction only. In bidirectional operation over a single optical fiber (FIG. 2), a first signal of a first wavelength band xcex1 is transmitted in one direction, and of a second signal of a second wavelength band xcex2 is transmitted in the opposite direction. The European patent application 0 658 988 teaches a compensation system from FIG. 2 and the appertaining description, which has a circulator comprising four terminals that has a compensation line for each direction of transmission.
It is the object of the invention to put forth dispersion compensators for bidirectional operation.
This object is achieved by a bidirectional dispersion compensator for a first and a second optical signal that are transmitted over an optical fiber in opposite directions, comprising:
first filter-coupler elements for merging said first optical signal, which is transmitted over a first portion of said optical fiber in a direction of said dispersion compensator, and said, counter-directional, second optical signal, which is transmitted in a direction of said dispersion compensator over a second portion of said optical fiber thereby producing merged optical signals;
a circulator having a first terminal to which said merged optical signals are fed;
a compensation line having a reflector on one side and which is connected to a second terminal of said circulator, via which said merged optical signals are sent into said compensation line and fed back into said circulator as reflected, dispersion-compensated signals; and
second filter-coupler elements to which said dispersion-compensated signals that are fed to a third terminal of said circulator, said second filter-coupler elements configured for separating first and second dispersion-compensated optical signals from said dispersion-compensated signals and for feeding said first dispersion-compensated optical signal into said second portion of said optical fiber and for feeding said second dispersion-compensated optical signal into said first portion, so that said first and said second dispersion-compensated optical signals are respectively forwarded in their previous directions.
This object is also achieved by a bidirectional dispersion compensator for a first and a second optical signal which are transmitted over an optical fiber in opposite directions, comprising:
a first circulator having a first terminal to which said first optical signal is fed via a first portion of said optical fiber and a first filter-coupling element;
a second circulator having a first terminal to which said second, counter-directional, optical signal is fed via a second portion of said optical fiber and an additional filter-coupler element;
a compensation line having a reflector on one side and which is connected to a second terminal of respective said circulators via a respective terminal of a further additional filter-coupler element, by which said first and second optical signals are merged via said further additional filter-coupler element, and reflected, dispersion-compensated signals are separated, with a dispersion-compensated first signal being fed into said second terminal of said second circulator and via a third terminal of said second circulator and via said first filter-coupler element into said second portion, and with said dispersion-compensated second signal being fed into said second terminal of said first circulator and via a third terminal of said first circulator and said additional filter-coupler element into said first portion of said optical fiber, so that said two signals are respectively forwarded in their previous directions.
This object is also achieved by a bidirectional dispersion compensator for a first and a second optical signal that are transmitted over an optical fiber in opposite directions, comprising
a circulator comprising a first, second, third, and fourth terminal, at whose first terminal a first optical signal that is transmitted in a direction of said dispersion compensator is fed in, and at whose third terminal a second optical signal that is transmitted in said direction of said dispersion compensator is fed in;
a filter-coupler element, which is connected to said second and fourth terminal of said circulator, said filter-coupler element having an additional terminal that conducts both optical signals;
a compensation line, which is connected to said additional terminal of said filter-coupler element and which terminates at an other end with a reflector, so that after traversing said compensation line and said filter-coupler element in said forward and reverse directions and refeeding into said circulator via said third terminal, a first dispersion-compensated optical signal and a second dispersion-compensated optical signal are fed into said optical fiber via said fourth terminal of said circulator, where said first dispersion-compensated optical signal and said second dispersion-compensated optical signal are transmitted further in their previous direction respectively.
One particularly advantageous soluction is given in the independent claim 1, which requires only a single compensation line which is equipped with a reflector R to compensate both optical signals. The two optical signals are merged by filter-coupler elements and are fed into the compensation fiber via a circulator. The reflected optical signals are separated via additional filter-coupler elements and are fed into the optical fiber as dispersion compensated signals. The outlay is greater than that for a unidirectional dispersion compensator only by a few filter-coupler elements.
In order to compensate the attenuation of the dispersion compensator and the transmission paths, optical amplifiers can be inserted. If separate amplifiers are provided for both input signals and both output signals of the dispersion compensator, an individual amplification control and correction of the amplitude curve can be performed for each signal. When this is not necessary, in a development of the invention the merged input and output signals can be amplified. The amplifiers must have a larger bandwidth in this case.
In a variant of the invention that makes use of two circulators, only three filter-coupler elements are needed. In this variant, it is also advantageous when bidirectional amplifiers are arranged in the feeds to the compensation line, so that different amplifications can be set for each signal. The compensation line can then be constructed as a fiber amplifier, as well.
An embodiment that is particularly advantageous utilizes a circulator with four terminals and a filter-coupler element for merging and separating the two optical signals and requires only one compensation line.
It is also advantageous to use an additional compensation sub-line. In this way, different compensation requirements that are conditioned by the different wavelengths can be taken into account. Different reflectors can also be connected via an additional filter-coupler element.